1.Field of the Invention
The present invention relates to an image forming apparatus that can copy a document by moving an exposure unit, and that can move a document that is to be copied.
2. Related Background Art
There are two types of automatic paper feeding devices: a recirculating document feeding (RDF) type and an automatic document feeding (ADF) type. An RDF device completes an exposure as a document is passing through an exposure unit, and discharges it upward (or downward) of the location of a portion for positioning an original, so that the device is capable of copying multiple sets by sequentially recirculating the document.
Compared with in a fixation reading mode in which the movement of a document is temporarily halted and the exposure position is shifted along the document area, there is no time loss, which is equivalent to the time required for shifting the exposure unit, for the RDF system in a flow reading mode in which an exposure is completed while a document is moving. This contributes the reduction of the time required for the exchange of a document, and constitutes a technical improvement for a high speed copying system that provides higher productivity. Further, when the above described mode is compared with the fixation mode at a predetermined time required for the exchange of a document, the RDF system in the flow reading mode can circulate a document at a lower shifting speed and can cause less noise.
By referring to the example of the employment of the flow reading mode and the RDF apparatus, there is also a system that performs high speed copying and provides high productivity by employing a flash exposure technique and the flow reading mode. Although a detailed explanation for the flash exposure technique is not given here, unlike the above exposure unit, an exposure unit that employs this technique is so designed that it completes the exposure of all the faces of a document at the same time, and requires a high power light source and a belt-shaped photosensitive body, which increases the size and the manufacturing cost of, and the power that is consumed by the apparatus. An apparatus that employs this technique is therefore limited to only certain of the large apparatus that performs high speed processing.
The second type, the ADF apparatus, fixes an original document in a predetermined position on a platen, and an exposure unit reciprocates above the document several times to expose the area to be copied and then discharges the document into a discharge tray. The ADF apparatus provides multiple copies of a document by repeating the above described process. Since such a copying system is employed together with a sorter that is provided on the output side, and as a document does not have to be repeatedly circulated, the document will not be heavily damaged.
While the above described conventional techniques are presently employed, various other techniques have been proposed that will provide higher speed processing with less noise production. As a first example, a sheet feeding device automatically selects, in consonance with a document size or a mode, one of two paper feeding means, which are provided at both ends of a glass plate, and feeds a document that is positioned on the glass plate to an image reader of a copying apparatus in order to form an image (U.S. Pat. No. 5,629,763).
According to a system in which a document can be recirculated, as is shown in FIG. 21, in a document fixation reading mode (in which paper is fed from a first paper feeding means), a plurality of originals P that are stacked on a sheet tray 304 are separated into individual sheets by a first separation means 306 and are fed by a feeding means 338. Thereafter they are sequentially mounted at an arbitrary position on a platen 303. Then, an image reader 380 (optical system) in a copying apparatus 301 moves in the directions indicated by the double-ended "a" arrow to read an image. Following that, the original P is mounted on a discharge means 311. In a document flow reading mode (in which paper is fed from a second paper feeding means), a plurality of originals P that are stacked on the document tray 304 are separated into individual sheets by a second separation means 314 and are fed by a paper feeding means 315. The optical system 380 is fixed at a position at a distance L from its home position (indicated by broken line), and an image on the original P is read while the original P is being fed at a constant speed along a wide belt 307 above the platen 303. The original P is then discharged by the discharge means and stacked on the sheet tray 304.
To select and to perform the flow reading mode process, a user mounts originals having a small size (A4, B5, LTR, etc.) on the document tray 304, and depresses a start key (not shown) of a copying apparatus to set a first entrance sensor 323 on and to set a sheet material length detector 368 off. First, the document tray 304 descends to a predetermined position by rotating around a support point 340. The bundle of originals P are transferred to the second separation means 314 side and are shifted until they are positioned at a bundle conveying position sensor 328 (the sensor 328 is turned on). The feeding of the original P to the platen 303 is begun by the second paper feeding means 315. The copying of the original P is performed according to the above described flow reading mode, and the original P is then discharged by a discharge roller 311 onto the document tray 304. As each of the originals P is discharged, its trailing end is pushed toward the second separation means 314 side by a stopper 321 to align the edges of the stacked originals. When the document is circulated once more, a bundle consisting of all the originals that were discharged is moved into position and the feeding and copying processes are again performed.
As a second example, there are patents (e.g. U.S. Pat. No. 5,799,237) for an automatic feeding apparatus, i.e., an ADF apparatus. This apparatus is so designed that an exposure unit is fixed at a predetermined position below a platen to perform flow reading while a document is conveyed above the platen and is then discharged onto a discharge tray. To make multiple copies, the document is not discharged after the flow reading is over, but is temporarily held at a position where the flow reading is completed. The exposure unit reciprocates a number of times that is equivalent to a required copy count, and thereafter the document is discharged to a discharge tray. In this manner, an apparatus of this type employs a mixed mode of flow reading and fixation reading to provide multiple copies.
Since the first conventional apparatus example is an image forming apparatus of a document recirculating type, high speed changing of documents is possible in the flow reading mode, and this apparatus is appropriate for attaining a high productivity and for reducing noise. However, the document must circulate a number of times that is the equivalent of a copy set count in order to prepare multiple copies. Thus, this apparatus causes heavy damage to an original document: stress acts on the document while it is passing through a separation means, and paper is curled while it is transferred along a curved path. In addition, feeding errors during the handling of the originals tend to occur, so that the above apparatus is not desired as a system that has high reliability.
The second conventional apparatus example is a type where a document does not recirculate, and does not cause damage to a document. However, taking into consideration the higher speed that is required for changing the originals and the accuracy in positioning (registration) of a document and a transfer sheet, during the use of the apparatus many problems arise.
For example, in the second apparatus example shown in FIG. 19, the reading locations in the flow reading mode are provided at a position 1002 or 1003. When the flow reading is to be performed for a sheet of paper having a small size, and when a copy set count is "1", distance Lx between an leading edge sensor 1004 and a flow reading position is long, so that the time that is required for changing the document is extended.
More specifically, since, in a mixed mode for flow reading and fixation reading, a wide belt 1005 cannot move while a preceding original is fixed in a position and is being read by the shifting of an optical system, the leading edge of a succeeding original must be held upstream from the leading edge sensor 1004. Therefore, the distance Lx is increased and more time is required to change a document. If the distance Lx between the leading edge sensor 1004 and the flow reading position is shortened in order to improve productivity, the rear edge of the document is affected by the turning off or on of the clutch of an upstream drive system (at the time of the paper separation or the paper feeding) while the flow reading is performed. This causes image blurring and the reliability of image forming is degraded. Scanning of a large size sheet of paper cannot be performed, and the apparatus will not function satisfactorily as a copying system. The apparatus shown in FIG. 20 has the same problems.